For optical fibers, the refractive index profile is generally classified according to the appearance of the graph of the function that associates the refractive index with the radius of the optical fiber. Conventionally, the distance r to the center of the optical fiber is shown on the x-axis, and the difference between the refractive index (at radius r) and the refractive index of the optical fiber's cladding is shown on the y-axis. The refractive index profile is referred to as a “step” profile, “trapezoidal” profile, “alpha” profile, or “triangular” profile for graphs having the respective shapes of a step, a trapezoid, an alpha, or a triangle. These curves are generally representative of the theoretical or set profile of the optical fiber. Constraints in the manufacture of the optical fiber, however, may result in a slightly different actual profile.
An optical fiber conventionally includes an optical fiber core, which transmits and/or amplifies an optical signal, and an optical cladding, which confines the optical signal within the core. Accordingly, the refractive index of the core nc is typically greater than the refractive index of the cladding ng (i.e., nc>ng). The propagation of an optical signal in a single-mode optical fiber may be broken down into (i) a fundamental mode (i.e., dominant mode) in the core and (ii) secondary modes (i.e., cladding modes) guided over a certain distance in the core-cladding assembly.
Conventionally, step-index fibers, also called single-mode fibers (SMFs), are used as line fibers for optical fiber transmission systems. These optical fibers exhibit chromatic dispersion and a chromatic dispersion slope meeting specific telecommunications standards, as well as standardized cut-off wavelength and effective area values.
To facilitate compatibility between optical systems from different manufacturers, the International Telecommunication Union (ITU) defined a standard reference ITU-T G.652, with which a standard optical transmission fiber (i.e., a standard single-mode fiber or SSMF) should comply. The ITU-T G.652 recommendations and each of its attributes (i.e., A, B, C, and D) are hereby incorporated by reference.
For a transmission fiber, the G.652 standard recommends: (i) at a wavelength of 1310 nanometers, a mode field diameter (MFD or 2W02) of between about 8.6 microns and 9.5 microns with a tolerance of ±0.6 micron (i.e., a MFD of between about 8.0 microns and 10.1 microns); (ii) a cable cut-off wavelength (λcc) of 1260 nanometers or less; (iii) a zero dispersion wavelength (λ0 or ZDW) (i.e., the chromatic dispersion coefficient) of between about 1300 and 1324 nanometers; and (iv) a chromatic dispersion slope of 0.092 ps/(nm2·km) or less at the zero dispersion wavelength. Conventionally, the cable cut-off wavelength (λcc) is measured as the wavelength at which the optical signal is no longer single mode after propagation over 22 meters of optical fiber, such as defined by subcommittee 86A of the International Electrotechnical Commission in the IEC 60793-1-44 standard, which is hereby incorporated by reference.
Increasing the effective area of a transmission fiber typically reduces its nonlinear effects. A transmission optical fiber having an enlarged effective area allows transmission over longer distances and/or increases the operating margins of the transmission system. Typically, standard single-mode fibers (SSMFs) have an effective area Aeff of about 80 μm2 (i.e., about 80 square-microns).
To increase the effective area of a transmission optical fiber, fiber profiles having an enlarged and flattened core compared with a SSMF have been proposed. Such an alteration in the shape of the optical fiber's core, however, typically leads to an increase in microbending losses as well as an increase in the optical fiber's effective cut-off wavelength (λCeff) and cable cut-off wavelength (λcc). Conventionally, the effective cut-off wavelength (λCeff) is measured as the lowest wavelength at which the optical signal is single mode after propagation over two meters of optical fiber as defined by subcommittee 86A of the International Electrotechnical Commission in the IEC 60793-1-44 standard, which, as noted, is incorporated by reference.
U.S. Pat. No. 6,658,190, which is hereby incorporated by reference, describes transmission optical fibers with an effective area enlarged to greater than 110 μm2. These optical fibers have a wide core—1.5 to 2 times that of a SSMF—and a configuration with a constant or shallowly depressed cladding. To compensate for the increase in microbending losses caused by an increase in the effective area, this patent proposes increasing the optical-fiber diameter. Increasing the diameter of the optical fiber, however, increases manufacturing costs and leads to cabling problems because of incompatibility with other optical fibers. Additionally, this patent indicates that the cut-off wavelength decreases with the length of the optical fiber under consideration and that the optical fiber achieves a single-mode character after one kilometer of transmission. Such a measurement of the cut-off wavelength, however, does not comply with the aforementioned standardized measurements. The optical fibers described in this patent have cable cut-off wavelengths greater than 1260 nanometers and zero chromatic dispersion wavelengths λ0 (or ZDW) of less than 1300 nanometers. The optical fibers of this patent, therefore, do not comply with the recommendations of the G.652 standard.
U.S. Pat. No. 6,516,123, which is hereby incorporated by reference, describes optical fibers having an effective area greater than 100 μm2 at a wavelength of 1550 nanometers. The optical fibers, however, have cable cut-off wavelengths that are greater than 1260 nanometers, which is noncompliant with the G.652 standard.
U.S. Pat. No. 7,076,139, which is hereby incorporated by reference, describes an optical fiber having an effective area of 120 μm2 at a wavelength of 1550 nanometers. This optical fiber, however, has a cable cut-off wavelength greater than 1260 nanometers and a ZDW of around 1280 nanometers, and therefore does not meet the G.652 standard.
U.S. Patent Publication No. 2005/0244120, which is hereby incorporated by reference, describes an optical fiber with an effective area of 106 μm2 at a wavelength of 1550 nanometers. This optical fiber, however, has a cut-off wavelength of 1858 nanometers, which is well above the limit imposed by the G.652 standard.
European Patent No. 1,477,831, and its counterpart U.S. Pat. No. 6,904,218, each of which is hereby incorporated by reference, describe examples of optical fibers having an effective area greater than 100 μm2 at a wavelength of 1550 nanometers. The example of FIG. 8 shows an optical fiber having a cut-off wavelength less than or equal to 1270 nanometers, but with a ZDW of 1295 nanometers (i.e., calculated from the fiber profile), which is outside of the G.652 standard.
U.S. Pat. No. 6,483,975, which is hereby incorporated by reference, describes an optical fiber having an effective area greater than 100 μm2 at a wavelength of 1550 nanometers. The cut-off wavelength values of this optical fiber, however, are too high to meet the G.652 standard.
Commonly assigned European Patent No. 1,978,383, which is hereby incorporated by reference, describes optical fibers having an effective area greater than 120 μm2, but a cut-off wavelength higher than 1260 nanometers and a zero chromatic dispersion wavelength and dispersion slope at the zero chromatic dispersion wavelength that do not meet the G.652 standard.
In brief, none of the foregoing patent documents discloses an optical fiber that possesses an effective area greater than 100 μm2 at a wavelength of 1550 nanometers while meeting the constraints of the G.652 standard with respect to cable cut-off wavelength, zero chromatic dispersion wavelength, and chromatic dispersion slope at the zero dispersion wavelength.
Therefore, there is a need for a transmission optical fiber that has an expanded effective area (i.e., greater than or equal to 100 μm2 at a wavelength of 1550 nanometers), and that has a cable cut-off wavelength less than or equal to 1260 nanometers, a zero chromatic dispersion wavelength of between 1300 nanometers and 1324 nanometers, and a chromatic dispersion slope at the zero chromatic dispersion wavelength less than 0.092 ps/(nm2·km). In other words, there exists a need for a single-mode optical fiber having an expanded effective area that, except for its mode field diameter, meets the ITU-T G.652 recommendations.